1. Field of the Invention
The invention relates to a device for drawing a web into a machine adapted to process the web, and more specifically to such a device for pulling a web of paper into a webfed rotary printing press. The press has at least two printing units with register rolls around which the paper is run, a pull-in bar to be detachably connected to the leading end of the web, and traction elements of finite length running over bend pulleys arranged adjacent to the side frames of the press, such elements extending along the full path along which the web is to be drawn into the press for printing on it so that the ends of the traction elements at one end of the press are attached to a driven winch drum at one end of the draw-in path and the opposite ends of the elements are attached to a further driven winch drum at the other end of the press. The traction elements may be wound and unwound from the two respective winch drums to move the pull-in bar through the machine along the path to be followed by the web.
2. Description of the Related Art
A paper web draw-in device of this type has been proposed in the prior art in which the winch drums arranged at the front and rear ends of the pull-in path have individual hydraulic drive motors which are so operated that it is only the drive motor, that is to the front in the direction of draw, that is activated. The rear drive motor at the other end of the press is just caused to rotate by the front drive motor and acts as a brake. During printing the two motors brake the traction elements. Accordingly in this known arrangement a heavy strain is built up in the traction elements since in the draw-in operation not only friction acts on the tractive elements since tension has to be exerted on the pull-in bar, but in addition the forces necessary to override the dragging effect of the opposite drive motor acts on the tractive elements, and such overriding force may be very substantial in the case of a hydraulic motor in the form of hysteresis. The sample applies for regular running of the press in which, owing to the motion of the register rolls, the braking effect exerted on the tractive elements also results in an increase in the strain on them. This heavy strain in the tractive elements however not only means that they are likely to snap and to cause injury to pressmen, but also that the register roll control system is heavily loaded and this may lead to failure to properly register. It is in fact to be assumed in this connection that the register rolls, around which the paper web is trained, have to be fitted with bend pulleys at their ends for the tractive elements so that the servo drive of the register rolls will be biased in one direction and motion in the other direction will be opposed. In the prior art the consequence of this has been the necessity of providing oversized servo drives for the register rolls and this is a factor having a generally unfavorable effect on the overall costs and complexity of the printing press.
Another remedy attempted in the prior art has been the provision of a tractive element take up and pay off device reacting to any change in the setting of the register rolls leading to a change in the free length of the tractive elements. However such a system greatly increases the general complexity of the press, since a substantial length of tractive elements has to be taken up and released when the register rolls are displaced.